[llvm] [ADT] Fix signed integer overflow (PR #155826)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-clang-aarch64-darwin` running on `doug-worker-4` while building `llvm` at step 4 "cmake-configure".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/190/builds/26318

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 4 (cmake-configure) failure: cmake (failure)
Upon execvpe b'cmake' [b'cmake', b'-DCMAKE_C_COMPILER=clang', b'-DCMAKE_CXX_COMPILER=clang++', b'-DCMAKE_BUILD_TYPE=Release', b'-DLLVM_BUILD_TESTS=ON', b'-DLLVM_CCACHE_BUILD=ON', b'-DLLVM_ENABLE_ASSERTIONS=ON', b'-DLLVM_INCLUDE_EXAMPLES=OFF', b'-DLLVM_LIT_ARGS=--verbose --timeout=900', b'-DLLVM_TARGETS_TO_BUILD=AArch64', b'-DLLVM_ENABLE_PROJECTS=llvm;cross-project-tests;lld;clang-tools-extra;clang', b'-GNinja', b'../llvm-project/llvm'] in environment id 4365123904
:Traceback (most recent call last):
  File "/Library/Python/3.9/site-packages/twisted/internet/process.py", line 433, in _fork
    self._execChild(path, uid, gid, executable, args, environment)
  File "/Library/Python/3.9/site-packages/twisted/internet/process.py", line 504, in _execChild
    os.execvpe(executable, args, environment)
  File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.9/lib/python3.9/os.py", line 583, in execvpe
    _execvpe(file, args, env)
  File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.9/lib/python3.9/os.py", line 616, in _execvpe
    raise last_exc
  File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.9/lib/python3.9/os.py", line 607, in _execvpe
    exec_func(fullname, *argrest)
FileNotFoundError: [Errno 2] No such file or directory: b'/opt/homebrew/bin/cmake'

```

</details>

https://github.com/llvm/llvm-project/pull/155826